This invention relates to electric machines in general and more particularly to synchronous machines having a field winding which is cooled to a low temperature.
The use of superconducting windings in electric machines permits a substantial increase of the flux in the air gap between the rotor and the stator of the machine. Futhermore, higher current densities may be obtained using superconductors with winding losses essentially eliminated. The ratio of power to volume and weight in such machines utilizing superconducting field windings is thus considerably higher than in a machine of conventional design. As described in the "Proceedings IEEE," Vol. 61 (1973), pages 112 to 115, a known turbo generator has a stationary outer armature and a revolving superconducting field winding. Since the field winding must be kept at a very low temperature, good thermal insulation of the rotor is provided, the insulation being achieved through the use of a high vacuum. To accomplish this an evacuated cylinder is required. The outer wall of the stationary vacuum cylinder carries a stator winding. The core of the rotor is formed by a tube of austenitic steel, on which the superconducting wires are arranged. The individual layers of the windings are held together to counteract centrifugal and magnetic forces using glass fiber and plastic tapes. The winding package so formed is further enclosed by a vacuum tight steel tubular enclosure.
Because of the differential thermal contraction of the materials used in the rotor and windings, displacement or deformation of individual conductors in the windings cannot be prevented after the rotor is cooled down. As is well known by those skilled in the art conductor movements of this nature are related to the generation of mechanical friction and deformation heat along with local flux changes. This can lead to at least a portion of the field winding going into a normally conducting state.
Another generator using a superconducting field winding is disclosed in U.S. Pat. No. 3,679,920. In the arrangement disclosed therein, the rotor comprises a support cylinder of non-magnetic material such as stainless steel. The individual conductors of the field winding are arranged either in slots cut on the outside surface of the support cylinder as shown on FIG. 2 or on the inside of a support cylinder using a hardenable plastic as shown on FIG. 5. However, windings of this nature on the inside of the support cylinder are difficult to manufacture. Furthermore, difficulties arise in attempting to cool these windings and the danger exists that, should a portion of the winding become normally conducting, the heat produced will not be removed fast enough and the entire winding will become normally conducting.
In view of these difficulties with the prior art arrangements the needs for an improved rotor for such an electrical machine in which the field winding is easier to manufacture, deformation of the conductors of the winding is eliminated during cooling and operation, and efficient cooling of the windings takes place, is evident.